Move unstructured conversion into pkg/runtime
Move unstructured conversion into pkg/runtime

Scheme conversion should support unstructured conversion natively to
allow going from unstructured to typed and back. It is not a higher
level responsibility to do that conversion because the scheme is the
only one who knows what types it supports.

Kubernetes-commit: 557f9ddfe6300ecdbdd70ea67dca63bf6b710ea7

kubernetes/kube-aggregator,kubernetes/kube-aggregator